Fix bar chart labels X-position calculations

[rosco54]

New Pull Request

Although this change fixes the example in the issue, and all other samples in the package and my private collection appear to render correctly, it will have to be proven in use. I've attempted to base all computational changes on those first principals that the original code follows, as far as I understand them.

Fixes #4558

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes